In this competitive renewal application of this Program Project grant now in its 30th year, multicjisciplinary approaches are employed to address the central theme of gene regulation in developing red blood cells. The premise underlying this work is that progress in the fundamental biology ofthe erythroid cell will provide critical, new knowledge that will have a positive impact on the management ofthe major red cell disorders, ncluding the hemoglobin disorders (sickle cell anemia and p-thalassemia), as well as congenital and acquired anemias. The Program utilizes multiple vertebrate systems (mouse, zebrafish, and human), contemporary genetics and cell biology, and emerging high-throughput chemical and shRNA screening procedures. Extensive interaction between the Project Leaders ensures intellectual vitality and synergy in execution of the proposed studies. The current proposals rest on exciting recent advances by the Project Leaders. In Project 1 (H. Lodish, Project Leader) new transcription factors, RNA polymerase II elongation factors, and chromatin-modifying enzymes important for terminal red cell development will be characterized. In Project 2 (S. Orkin, Project Leader) strategies will be developed to translate recent break-though findings in the study of fetal hemoglobin switching in humans to effective platforms for the discovery of new pathways or drugs suitable for directed reactivation of HbF in adult with hemoglobin disorders. In Project 3 (L. Zon, Project Leader) a newly discovered aspect of erythroid cell gene expression, transcriptional elongation, will be explored further in both the mouse and zebrafish through the study of conditional mutant mice and the role ofthe HEXIM complex in erythroid development. In Project 4 (A. Cantor, Project Leader) molecular approaches will be utilized to delineate the determinants that distinguish gene activation versus repression for the master erythroid factor GATA-1 during erythroid cell maturation. In Project 5 (B. Paw, Project Leader) new components ofthe complexes required for proper iron and heme trafficking in developing red cells will be characterized in both the zebrafish and mouse. Each ofthe Projects in this Program Project grant addresses important aspects of the expression program and function of erythroid cells Findings from these studies will provide new opportunities for the design of novel approaches to the management of red blood cell disorders.